The Republican chairman of the Senate Judiciary Committee recommended Friday that the Justice Department investigate for possible criminal charges the author of the now-famous dossier alleging the Trump campaign coordinated with the Kremlin during the 2016 election.

The move by Sen. Charles E. Grassley (R-Iowa) marks a major escalation in conservatives' challenges to the FBI's credibility as the agency investigates whether any Trump associates committed crimes. Another Republican, Sen. Lindsey O. Graham (R-S.C.), joined in the letter to the Justice Department.

Their letter makes what is called a criminal referral to the Justice Department, suggesting it investigate the dossier's author, former British spy Christopher Steele, for possibly lying to the FBI. It is a crime to lie to FBI agents about a material fact relevant to an ongoing investigation.

The letter says Steele may have lied to the FBI "regarding his distribution of information contained in the dossier."In response to a lawsuit filed in London, Steele has said he had off-the-record conversations with a handful of news organizations during that period, including The Washington Post. It was not immediately clear how conversations with reporters might be materially relevant to the investigation the FBI was conducting.

The move by Grassley and Graham was viewed skeptically by some Democrats and Republicans on the Judiciary Committee, as well as experts in criminal law.

Steele's role has become a matter of increasingly heated debate, with the White House and Republicans on Capitol Hill suggesting his reports were biased, since he was hired with money that ultimately came from the Hillary Clinton campaign and the Democratic National Committee.

Steele's allegations — in addition to other information, including intelligence intercepts and an Australian diplomat's account of a conversation with a Trump adviser in a London wine bar — prompted the FBI to open a counterintelligence investigation into Trump associates in July 2016, according to current and former officials. The Australian warning was first reported by the New York Times.

That investigation has since expanded into a special-counsel probe by Robert S. Mueller III, whose work has led to indictments of two former Trump campaign officials, as well as guilty pleas and cooperation deals from two others. As the Russia probe has heated up, some Republicans have amplified their attacks on Steele's dossier, contending it was a weapon in a political smear campaign they claim was aided by the FBI.

Steele did not respond to a request for comment. But a lawyer for the consulting firm that hired him, Fusion GPS, had a strong reaction.

"After a year of investigations into Donald Trump's ties to Russia, the only person Republicans seek to accuse of wrongdoing is one who reported on these matters to law enforcement in the first place," the lawyer, Joshua A. Levy, said in a statement. "Publicizing a criminal referral based on classified information raises serious questions about whether this letter is nothing more than another attempt to discredit government sources, in the midst of an ongoing criminal investigation. We should all be skeptical in the extreme."

A veteran prosecutor, Peter Zeidenberg, said he had never heard of anything like the Grassley-Graham complaint and labeled it "nonsense" designed to detract from ongoing inquiries into Russian interference in the 2016 election.

"The FBI doesn't need any prompting from politicians to prosecute people who have lied to them," said Zeidenberg, a federal prosecutor for 17 years. While members of Congress make criminal referrals from time to time, they are usually related to independent congressional investigations, not to material already known to the FBI. "They should stay in their lane," Zeidenberg said of the Grassley-Graham effort.

Another former federal prosecutor, Justin Dillon, said it was too early to assume the letter was simply a political attack. "Given that we don't know what the basis for the referral is, it seems premature to simply assume it was partisan," Dillon said. "I do find it hard to believe that Grassley and Graham would just make this up out of whole cloth."

Grassley said in a statement Friday, "If we see what appears to be credible evidence of a violation that should be further investigated based on information from any source, public or non-public, we are going to formally notify the department."

The top Democrat on the Judiciary Committee, Sen. Dianne Feinstein (D-Calif.), said she and other Democrats had not been consulted about the request for a criminal inquiry.

"This referral is unfortunate," Feinstein said in a statement. "It's clearly another effort to deflect attention from what should be the committee's top priority: determining whether there was collusion between the Trump campaign and Russia to influence the election and whether there was subsequent obstruction of justice."

One Republican aide familiar with Judiciary Committee operations said: "It's pretty clear that Grassley and Graham are interested in carrying water for the White House, but that is not reflective of the whole committee. There are members of Judiciary who are upset with Senator Grassley for carrying it so close to the vest." The aide asked not to be identified because he was not authorized to speak publicly on the matter.

Steele was a senior intelligence officer monitoring the Kremlin for Britain's intelligence service, MI6, before he left the agency and established a consulting firm, Orbis. In that role, he worked for a range of clients, including law firms and Fusion GPS.

The dossier is a compilation of reports he wrote for Fusion GPS between June and late October 2016, plus a final report in December, after the election. Among other things, the dossier alleged that the Russian government collected compromising information about Trump and that the Kremlin was engaged in an effort to assist his campaign for president.

U.S. intelligence agencies later released a public assessment asserting that Russia interfered in the 2016 election to aid Trump. The FBI has been investigating whether Trump associates helped the Russians in that effort.

Trump has vigorously denied the allegations in the dossier and has called the FBI probe a witch hunt.